C\C++语言中的计时函数

1. 中函数clock()，返回类型clock_t，精确度，毫秒级别

#include  
#include  
#include  

void test() 
{ 
	int i = 0; 
	int j = 0; 
	double a = 0; 

	while (i++ < 1000000) 
		while (j++ < 1000000) 
		{ 
			a = sqrt(2.0); 
		} 
} 

int main(void) 
{ 
	clock_t start, finish; 
	double  duration = 0.0; 

	start = clock(); 

	test(); 

	finish = clock(); 
	duration = (double)(finish - start); //输出单位ms
	duration = (double)(finish - start) / CLOCKS_PER_SEC; //输出单位为s, //#define CLOCKS_PER_SEC 1000 
	
	printf("%f seconds\n", duration);  

	return 0; 
} 

2. 最精确的计时：QueryPerformanceCounter来查询定时器的计数值，如果硬件里有定时器，它就会启动这个定时器，并且不断获取定时器的值，这样的定时器精度，就跟硬件时钟的晶振一样精确的。

#include  
#include  
#include  
#include 

void test() 
{ 
	int i = 0; 
	int j = 0; 
	double a = 0; 

	while (i++ < 1000000) 
		while (j++ < 1000000) 
		{ 
			a = sqrt(2.0); 
		} 
} 

int main(void) 
{ 
	LARGE_INTEGER start; 
	LARGE_INTEGER end; 
	LARGE_INTEGER freq; 

	QueryPerformanceFrequency(&freq); 
	QueryPerformanceCounter(&start); 

	test();  

	QueryPerformanceCounter(&end); 

	printf("user time : %.10f seconds\n", (double)(end.QuadPart - start.QuadPart) / (double)freq.QuadPart); 

	return 0; 
} 

3. 中函数time(&t)， 精确度，秒级别 

功能：取以秒为单位的，从1970年1月1日格林威治时间00:00:00算起的当前时间，并把它存在长整形变量t中，函数返回如前所述的时间秒值。 

#include  
#include  
#include  
#include  
#include 
int main() 
{ 
	time_t t; 
	time(&t); 
	printf("Today's date and time: %s",ctime(&t)); 

	time_t first, second; 
	first=time(NULL); 
	Sleep(2000); 
	second=time(NULL); 
	printf("The difference is: %f seconds",difftime(second,first)); 
	getch(); 

	return 0; 
}